This invention relates to methods and apparatus for monitoring structural members subjected to transient loads, more particularly, to monitoring and analyzing the frequency characteristics of a structural member vibrating in response to a transient load in order to measure changes in the structural integrity of the member and to classify transient loads by nature and type.
As used herein, the following terms are defined in a conventional manner: "acoustic" means relating to sound; "sound" means a vibratory disturbance in the pressure and density of a fluid or in the elastic strain in a solid; "vibration" means oscillation of a parameter (e.g., displacement, velocity, acceleration) that defines the motion of a mechanical system; "sonic" means sound with a frequency in the audible range of the human ear, between about 20 and 20,000 Hz.; "ultrasonic" means sound with a frequency above the audible range of the human ear; "infrasonic" means sound with a frequency below the audible range of the human ear; and "waveform" means the instantaneous amplitude of a signal as a function of time.
As used herein, a "structural member" is defined as a mass of material having an elasticity that is capable of transmitting sounds, including standing waves, corresponding to vibratory motion of the structural member in a direction along a selected dimension of the mass. A structural member is further defined as a mass that is acoustically isolated from adjacent structural elements sufficiently so that the sounds corresponding to the motion of the structural member can be discriminated from the sounds corresponding to the motion of other structural elements. Typically, a structural member has six dimensions of motion; three orthogonal translational dimensions--e.g., horizontal, vertical, and lateral--and three rotational dimensions--one dimension of rotation about each translational dimension.
As used herein, a "transient load" is defined as a stress-event caused by the application of a force (mass) to a structural member for a duration of time and which stimulates on the structural member to vibrate, thereby to transmit sounds corresponding to the impact of the transient load. A transient load may be applied to one or more specific locations on the structural member, or to the entire structural member.
Structural members are used in construction and have a certain load bearing capability that is related to the size, elasticity and yield strength of the member, which determine how much displacement or deflection the member can withstand without permanent deformation or other irreversible change in mechanical properties.
Structures exposed to repeated transient loads of the same or differing magnitude or duration, or are susceptible to defects caused by fatigue of the materials used, which defects affect adversely the load bearing capabilities, i.e., the structural integrity, of the structural member. Such defects may be localized structural defects such as cracks and fatigue fractures. Similarly, structures which are exposed to weather over extended periods of time are subject to defects caused by corrosion or oxidation, or general changes to the mass or its microstructure resulting from, for example, defective materials, defective materials processing or manufacturing techniques, excessive stress events over use, and the like. Significant defects can result ultimately in complete structural failure of the member even when it is subjected to a transient load that is within the original load bearing capability of the structural member. Accordingly, structural members are periodically inspected in an effort to discover any defects that might affect the structural integrity before a structure becomes unsafe for its intended purpose, so that the member can be repaired, replaced or taken out of service.
Highway and railway bridges are examples of structures composed of structural members that are subject to transient loading and exposed to the elements where failure of a structural member can have catastrophic consequences. There are approximately 600,000 bridges throughout the United States of widely differing age and condition. Many of these bridges are composed of multiple spans, where each span is supported by one or more support beams.
The number of bridge inspectors in many places is too few to inspect adequately all of these structures and their component structural members for safety and the degree of use (which affects expected life) at sufficiently frequent intervals. Furthermore, in many instances, visual inspections and conventional diagnostic measurements alone are inadequate to evaluate the safety of the structure, particularly to detect reliably fatigue cracks, fractures, corrosion or oxidation or other defects that are invisible to the human eye.
Heretofore, methods of monitoring structural members for defects and undesired changes in structural integrity and for safety inspection purposes have depended largely upon visual inspection, destructive testing, i.e., testing the load carrying ability of the design to failure and extrapolating the test results to a structure in actual use, and non-destructive testing, i.e., testing structural integrity of the member without destroying its functional utility. Non-destructive testing techniques include using ultrasonic stimulators to produce ultrasonic signals in the structure at frequencies of from about 100 kHz to about 500 kHz which can be used to detect defects, e.g., U.S. Pat. Nos. 4,598,592, 4,535,629, 4,397,186 and 4,188,830, using magnetic eddy current flow to detect anomalies in the magnetic field caused by physical defects in a structure, transmitting mechanical acoustic vibrations at a point on the structure surface to produce mechanical vibration reasonances that may be detected by a microphone, and by performing stress measurements by x-ray diffractometry, e.g., U.S. Pat. No. 4,561,062.
U.S. Pat. No. 4,609,994 refers to monitoring acoustical emissions of a structure placed under stress using a detector-analyzer unit wherein the detector is an acoustic transducer secured to each structural member for providing an output signal representative of any acoustic emission, e.g., a piezoelectric transducer at frequencies of from 0.1 to 2 mHz or an accelerometer transducer at frequencies of from 1 to 20 kHz, and the analyzer is at least one signal conditioner circuit coupled to the transducer output signal for providing at least one derivative signal having characteristics correlatable with preselected characteristics of the output signal of the acoustic detector, a measuring circuit coupled to the signal conditioner circuit providing, for each derivative signal, a digital output signal representing one of a set of emission parameters correlatable with the preselected characteristics of the output signal of the detector, and a microprocessor coupled to each measuring circuit that contains a set of base values for periodically comparing the set of base values with corresponding detecting emission parameters to determine the existence of a problem situation when base values are exceeded. The microprocessor is connected to the central control unit which receives warning signals and other data from a plurality of remote detector-analyzer units associated with a plurality of structures and is the center for coordinating a suitable response. The central control unit also can program and reprogram the remote detector-analyzer units.
U.S. Pat. No. 4,164,149 refers to using angular motion sensors including an electrically conductive element and a permanent magnet secured to a structure so that angular deflection of the structure will cause relative movement of the coil and magnet and cause a current to flow. A plurality of angular motion sensors can be located at selected locations of a composite structure to obtain an initial waveform signature of the composite structure and a ratio of the input signals causing the vibration to the measured angular deflection signals, can be obtained by comparing the amplitude, phase, and frequency characteristics typically using fast Fourier transform techniques, so that subsequent changes to the initial composite waveform signature, which indicate corresponding changes in the structural characteristics of one or more individual components can be determined.
U.S. Pat. No. 4,549,437 refers to providing each segment of a complex multiple segment strucute with at least one acoustic sensor, recording the intensity and frequency distirbution of the sensed acoustic waves, and comparing the sensed acoustic waves against either a standard or over time and/or from one segment to another segment of the complex structure. Acoustic sensors are described as piezoelectric sensors and frequencies below 100 kHz are filtered to be removed from the signal.
These techniques, however, have been of limited use in many structures because of their inherent complexity and expense in implementation, the labor intensive procedures of interrogating a structure and interpreting the results of the interrogation, and because their use is typically limited to structures of certain material compositions and cannot be broadly applied to support members of different compositions such as concrete.
Predicting the structural health of a structure also is limited by the known methods of determining the nature and type of transient loading on structures. For example, in the case of bridges, these latter methods include using persons to take infrequent surveys of the type and volume of traffic, using pneumatic tubes across roadways to count vehicles, using radar to determine the speed of vehicles, and weighing selected vehicles to develop vehicle-weight profiles to estimate the use and loading on a road or bridge. These techniques, while often useful, suffer from the necessity of having to imply results based upon analyses or computer models or the loads being experienced by the structure and are subject to error because they utilize a small sample of selected vehicle-weight data that may or may not correspond to actual usage of the particular structure. Further, the equipment used to obtain the information is subject to vandalism or damage during data acquisition and require substantial operator supervision to obtain usable data. Additionally, the existing methodology also fails to take into account factors which are known to have a significant effect on vehicle affects on a bridge, including, without limitation, the type of suspension of the vehicle and discontinuities in the bridge surface, e.g., potholes, bumps, debris, etc.
Considerable effort has been devoted to measuring and modeling the dynamic behavior of bridges or bridge components in response to transient loads imposed by vehicular traffic to aid in the determination of safe loading capabilities and to the design of safer bridges. Techniques for measuring the sound transmitted by and the sound generated by support beams of bridge spans of various types and under various conditions, are known. Strain gauges, deflectometers, accelerometers, and seismometers have been used in various combinations to detect stress, deflection or displacement, and acceleration of bridge support beams. Techniques for using microphones and other acoustic emission sensors to detect sounds generated by bridges either in response to an applied mechanical or acoustical force, or self-generated in response to a transient load, also are known.
A bridge span support member will respond to a passing vehicle by vibrating. The resulting sounds will be transmitted along its various dimensions. While the vehicle is on the support member, the member will vibrate at a fundamental and harmonic frequencies in accordance with the forced function of the combined mass of the structural member and the vehicle. After the vehicle has left the bridge, the support member will continue to vibrate at the natural fundamental and harmonic frequencies of the support member. These vibrations along a given direction or dimension thus form a characteristic structural acoustic signature of the member in the given direction in response to a variety of transient loads.
As used herein, the term "structural acoustic signature" means the variation, over time, of selected frequency amplitude peaks of the spectral plot of a structural member vibrating in its free mode, i.e., vibrations which occur subsequent to the transient load that stimulated the vibrations in a given dimension. The spectral plot is mathematically derived from the detected sound signals in the given dimension by performing a fast Fourier transform ("FFT") upon a sample of detected sound signals transmitted by or other vibrational information of the structural member being monitored.
The vibrational information is typically detected by accelerometers placed on the structural member to detect sonic and infrasonic frequencies. Different dimensions will have different lengths, and, hence, different natural frequencies and structural acoustic signatures, accordingly. Thus, a structural member may have a composite signature including more than one structural acoustic signature in more than one dismension.
It also is known from the bridge vibration studies that (1) the speed of a vehicle passing over a span affects the vibration pattern; usually the peak amplitude of vibration dynamic of motion increases with increasing vehicular speed, (2) the amplitude of the forced vibration also is a function of the impact of the vehicle, (3) the roughness of the approach to the bridge appears to affect the oscillations of the bridge more than the roughness of the surface of the bridge. It also is known that the suspension system and the number of axles and the spacing between axles of the vehicles and the axle and vehicle frequencies affect the nature of the motion of the bridge, and that when the natural resonant frequency of the axle or the vehicles is the same as that of the bridge, there is resonance resulting in increased amplitude of vibration of the bridge.
Notwithstanding years of development effort and study and the need for improved monitoring of and enhanced inspection techniques for determining the safety of structural members, including without limitation bridges, there is no commercial use of a method or apparatus for monitoring structural members subjected to transient loads that provides directly measured information regarding changes in the relative structural integrity and/or the nature of the transient loading of the structure having the advantages and benefits of the present invention.
It is, therefore, an object of this invention to provide direct measurement of changes in the structural integrity of a structural member. It is a further object to measure changes in the structural acoustic signature of a structural member in response to a transient load to identify changes in the structural integrity of the structural member. It is another object to obtain more than one independent measurement of the structural responses and to correlate the independent measurements, thereby to obtain enhanced determinations of any changes. It is a further object to measure such changes using transient loads that are normally applied to the structural member being monitored during use.
It is another object of the invention to measure changes in stiffness of the structural member based on changes in the structural acoustic signature of the member in response to transient loads.
It is another object of the invention to provide for monitoring structural members of bridge spans using sensors for detecting the sound transmitted by the structural member in response to a transient load in one or more dimensions and particularly using accelerometer sensor systems to detect sonic and infrasonic sounds.
It is another object of the invention to provide for analyzing the detected sounds to count the number of transient loads acting on the member.
It is another object of the invention to provide for analyzing the detected sounds to determine the duration and, hence, velocity of a transient load.
It is another object of the invention to provide for analyzing the detected sounds to determine the mass of a transient load.
It is a further object of the invention to measure and record the number of stress range events on a structural member to provide a history for fatigue calculations.
It is another object to identify and classify transient loads applied to structural members by accumulating counts of vibration waveforms corresponding to the motion of the support member as it is stimulated by a given transient load including the onset, the maximum deflection, and the withdrawal of the transient load, and after the withdrawal of the load, whereby vibration waveforms of unknown transient loads can be identified by comparision of one or more components of the unknown waveforms to a library of waveforms of known transient loads or to a plurality of threshold signals establishing the different categories of possible transient loads and identifying a category corresponding to the transient load.
It is a further object of this invention, in the context of highway bridges, to provide for identifying, counting, weighing, and determining the speed of moving vehicles crossing a structural member supporting a bridge span using accelerometer sensors and/or seismic accelerometer sensors for detecting sonic and infrasonic sounds.
It is a further object of this invention to provide an automated central station monitoring system for monitoring a plurality of remote structural members for change in their structural integrity and the nature and type of transient loading without requiring continuous operator supervision at the central monitoring station.